1. Field of the Invention
The present invention relates to a method and a device for measuring the distance and relative speed of multiple objects using an FMCW (frequency modulated continuous wave) radar, in which transmitted signals having linear time-frequency ramps are emitted and the signals reflected on the objects are received and mixed with the transmitted signals.
2. Description of Related Art
Published German patent document DE 42 42 700 describes a method for measuring the distance and speed of objects with the aid of electromagnetic waves, in which the frequency of a transmitted signal is modulated. The signals received during a rise and a fall of the frequency of the transmitted signal are mixed with the transmitted signal and the intermediate frequency signals resulting from the mixing are spectrally analyzed. The distance and speed of at least one object is computed from the frequency of spectral lines of the intermediate frequency signals during at least one rise and at least one fall of the frequency of the transmitted signal.
In this procedure, an intermediate frequency is obtained for each frequency ramp and each detected object; the frequency position of this intermediate frequency may be assigned to a combination of distance and relative speed values. The combination of these intermediate frequency signals and associated distance and relative speed values yields common points of intersection, which may be represented in a distance-relative speed diagram for example. These points of intersection represent the measured distances and relative speeds of the objects. When a plurality of objects is detected, for example two objects, a total of four points of intersection are obtained, two of these points of intersection representing real objects and two further points of intersection being apparent objects obtained as a result of ambiguities.
To be able to rule out such ambiguities, the FMCW radar method has been refined in such a way that a plurality of consecutive frequency rises and falls are strung next to one another, the frequency ramps having different slopes. However, even with this refined method, situations may actually arise in which such apparent objects are detected over a number of measurement cycles as a result of multiple detected objects and are processed as real objects, since these apparent reflections continue to be received over a long period of time. In particular, in traffic situations where a plurality of detected objects moves with the same dynamics, i.e., when the distances and relative speeds of the plurality of objects change only very slowly over time, apparent objects remaining unchanged may continue to be received over a number of measurement cycles, because these objects are recognized by tracking methods and processed as real objects, although they are apparent objects.